It is estimated that domestic phosphate reserves currently contain about 0.015% by weight of uranium as U.sub.3 O.sub.8 which corresponds to more than 600,000 tons of extractable uranium. Exploitation of the uranium in these reserves during the manufacture of phosphatic fertilizers provides industry with a unique opportunity to develop an alternate source of uranium, a metal of considerable industrial and strategic importance. Satisfactory commercial production of phosphatic fertilizers involves the production of wet-process phosphoric acid wherein phosphate rock is acidulated with a mineral acid such as sulfuric acid.
Several processes have been developed for effecting the selective recovery of uranium from wet-process phosphoric acid solutions. One such process is described in commonly assigned U.S. Pat. No. 3,711,591 issued Jan. 16, 1973 in the names of Fred J. Hurst and David J. Crouse. Inasmuch as the present invention is preferably used in conjunction with this patented process, the aforementioned patent is incorporated herein by reference. While the present invention is described herein as being practiced with the incorporated patent, it will appear clear that the present invention may also find application in other known processes used for the selective recovery of uranium from wet-process phosphoric acid solutions.
Generally, the process of the aforementioned patent provides a two-cycle procedure for extraction of uranium from wet-process phosphoric acid solutions by successive and selective manipulations of the uranium valence state to promote transfer of the uranium between the appropriate phases. In the first cycle, hexavalent uranium is removed from the phosphoric acid solution by extraction into a first mixture of organic solvents and then subjected to a reductive strip solution of phosphoric acid and ferrous [Fe(II)] ions dissolved therein in sufficient amount to facilitate reduction of uranium from the hexavalent to the tetravalent state. This reductive step increases uranium concentration by a factor of up to about 100. In the second cycle, the uranium-loaded reductive strip solution is contacted with a second mixture of organic solvents to transfer uranium to an organic phase from which it is stripped by contact with an ammonium carbonate solution to form a precipitated ammonium uranyl tricarbonate compound. This compound is thermally decomposed at effective temperatures to produce a U.sub.3 O.sub.8 product acceptable for uranium enrichment processes.
The preferred organic solvent for practice of the present invention is the organic solvent utilized in the above-described patent which is a synergistic solvent mixture of di(2-ethylhexyl) phosphoric acid (DEPA) and trioctylphosphine oxide (TOPO) dissolved in a high boiling aliphatic hydrocarbon diluent. As utilized hereinafter, reference to organic solvents shall mean a 0.5 M DEPA-0.125 M TOPO mixture dissolved in n-dodecane (NDD). Results comparable to those obtained herein for NDD in the practice of the present invention are expected for other aliphatic diluents such as kerosene and commercial solvent formulations. The subject method may also be applied to other organic solvents known in the art for uranium recovery. For example, other phosphonate and phosphine oxide mixtures have been described for such purposes in such publications as "Solvent Extraction of Uranium From Wet-Process Phosphoric Acid," by Fred J. Hurst, et al, ORNL/TM-2522, Oak Ridge National Laboratories, Oak Ridge, Tenn. (April 1969). Copies of the foregoing report may be purchased from the U.S. Department of Commerce, NTIS Center, Port Royal Road, Springfield, Va. 22161.
While the recovery of uranium from wet-process phosphoric acid solutions by the aforementioned patented process has been successful, some problems have developed in the practice of the process which led to the inability of the reductive strip stage of the process to effect adequate reduction of uranyl ion [U(VI)] to uranous ion [(U(IV)]. This deficiency has a significant impact on economic attractiveness of the process and impedes efficient uranium recovery.
In order to maintain adequate levels of reduction, the quantity of elemental or ferrous iron added to the reductive strip stage had to be significantly increased. This increased iron concentration, up to about 10 times the stoichiometric amount, was economically unattractive and also created severe operating problems in and downstream of the reductive strip stage. For example, the excess iron not removed in product streams as a contaminant accumulates as complex iron phosphates and cruds within process vessels and related equipment requiring frequent and undesirable downtime for maintenance. Solids accumulation has also been identified as one of the major causes of inordinate solvent losses by the formation of stabilized emulsions. Also, a significant amount of this excess ion may be introduced to the second cycle where it can contaminate the ammonium uranyl tricarbonate product to such a degree that it may be unsuitable without additional purification. In an effort to alleviate the foregoing problems, it has been suggested that conducting the reductive strip stage of the process in a controlled inert gas environment may reduce iron consumption and minimize solids accumulation. Implementation of this procedure, however, has been ineffective for controlling the aforementioned problems.